General a reboot once a week or so is a good way to just flush out the ram. I would worry about that on my PC but I general do something that requires a reboot. (installing drivers, changing some settings, screwing around in places that I should not be on windows) I figure if I had a mac I would general do about the same level of no-nos that would require a reboot.
As for stabilty the familys computer has been left up and running for 2+weeks straight with up to 4 users log in at one time not requiring a reboot (the 2 weeks where ended because a buggy game I like to play locked up on me and when I play that game I general exact to cold restart my computer a few times that night since it not the OS fault just poor coding for the game) General as long as it is desently maintain and not really messed with they can stay up and running for a long time. Same goes for a mac
As for stabilty the familys computer has been left up and running for 2+weeks straight with up to 4 users log in at one time not requiring a reboot (the 2 weeks where ended because a buggy game I like to play locked up on me and when I play that game I general exact to cold restart my computer a few times that night since it not the OS fault just poor coding for the game) General as long as it is desently maintain and not really messed with they can stay up and running for a long time. Same goes for a mac